They are just regular private keys. You could send coins to their address, and spend the coins. But nobody ever has, and it wouldn't be a good idea now that the private keys are less than private.

These private keys are used to add an extra signature to blocks staked by prime controllers to prove that they were staked by a prime controller, and so eligible for the higher block reward.

This means that while there are "only" 50 different PC keys, there's no limit on the number of wallets that can act as prime controllers. 1000 wallets can all use the same PC private key to prove that they are eligible for the higher rate. It also means that you can't really "sell" a PC any more than you can sell a Bitcoin private key - you can't prove that you have forgotten the private key.